The present invention relates to a transmission system and an optical splitting unit, and particularly to a wavelength division multiplexing (WDM) transmission system for transmitting signal lights of different wavelengths and an optical coupler/branching filter used for the system.
The recent growing traffic of data attributable to the prevalence of the Internet and the like necessitates the increase in the transmission capacity of an optical fiber cable. WDM and speed-up of transmission are possible schemes for the increase of transmission capacity.
The WDM system is designed to transmit a number of signal lights of different wavelengths based on multiplexing through an optical fiber cable. Based on the splitting and coupling of part of multiplexed signal lights, it becomes possible for an optical fiber capable of transmitting the signal lights not only between two places but among a number of places.
There is proposed, for a data rate of 2.4 Gbits/s, a method of splitting and coupling part of wavelengths at once by disposing an optical coupler/branching filter formed of passive parts between optical amplifiers. A technique relevant to this method is described in Japanese Patent Laid-open (Kokai) No. Hei 11-275007.
At a data rate of 10 Gbits/s which is four times the 2.4 Gbits/s, the influence of light dispersion on the transmission path is not negligible and the compensation against dispersion is required. A technique pertinent to the dispersion compensation is described in Japanese Patent Laid-open (Kokai) No. Hei 7-301831.
However, for the transmission at 10 Gbits/s on a transmission path of usual optical fiber (non-dispersion shifted fiber: NDSF), it is difficult for the above-mentioned conventional WDM technique to equip necessary dispersion compensating fibers (DCF). The transmission at 10 Gbits/s necessitates DCFs of different characteristics depending on the respective wavelengths to be coupled, split and passed through without being split.
Generally, the optical branching filter, optical coupler and DCF are optical parts which suffer large losses, and therefore using the optical coupler/branching filter and DCF in series results in an increased loss, making a long distance transmission difficult.
An object of the present invention is to provide an optical transmission unit which is capable of performing the coupling and branching of individual wavelengths at a data rate of 10 Gbits/s.
In order to achieve the above objective, the inventive optical coupler/branching filter is designed to set proper values of dispersion compensation depending on the state of transmission path.
The loss of signal light caused by using an optical coupler, an optical branching filter and DCF together is compensated by means of a plurality of low-excitation optical amplifiers, thereby enabling a long-distance transmission.
These and other objects and many of the attendant advantages of the invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.